How Tropical Storm Andrea Is Spinning Up Tornadoes

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Tropical Storm Andrea has spawned as many as six tornadoes as of
1 p.m. EDT today (June 6), with many more tornado warnings being
issued, according to various news reports. There is currently a
tornado watch for most of the state of Florida until 9 p.m. EDT,
and those watches could start to extend further north as the
storm moves inland.

Hurricanes and tornadoes are typically thought of as separate
phenomena, with tornadoes conjuring up images of the flat prairie
and hurricanes associated with the warm, coastal tropics.
Hurricanes are much, much larger than tornadoes, but tornadoes
are capable of producing much faster winds than hurricanes.
However, some tropical storms and hurricanes are capable of
spinning up tornadoes, as
Tropical Storm Andrea is doing.

But how do hurricanes and tropical storms create tornadoes?

Hurricanes and tropical storms, collectively known as
tropical cyclones, provide all the necessary ingredients to form
tornadoes. First, most hurricanes carry with them individual
supercells, which are rotating, well-organized thunderstorms.
(These are typically the storms that spin up monster twisters in
the Plains. All tornadoes need thunderstorms to form, said Brian
McNoldy, a researcher at the University of Miami.

Second, hurricanes bring with them warm, moist air, which acts as
their fuel. This creates instability in the atmosphere — namely,
a layer of warm air with slightly colder and less-moist air
above. This arrangement is unstable because the warm air wants to
rise, since it is less dense. [ 50
Amazing Hurricane Facts ]

Finally, hurricanes create wind shear, or an abrupt change in
wind speed and direction over a short change in height. These
alternating winds can create swirling air, called rolls. These
vortices may then be flipped vertically — creating tornadoes
— by thunderstorm updrafts, which are basically currents of warm,
rising air, McNoldy told LiveScience's OurAmazingPlanet.

Most hurricanes that make landfall create tornadoes, McNoldy
said. "It's pretty uncommon to not have tornadoes with these," he
said, adding that so far, the number of tornadoes spawned by
Tropical Storm Andrea is not unusually high. Tornadoes mostly
form over land, instead of over water, because the land slows
down surface-level winds, creating even more wind shear, McNoldy
said. Tornadoes form wherever these pre-existing supercells
happen to be, he added, but meteorologists are still unable to
predict exactly where tornados will strike.

These twisters usually form in the swirling bands of rain outside
the cyclone, typically in the "front-right quadrant" of the
storm, McNoldy said. In other words, if the storm is moving
north, you're most likely to find tornadoes to the northeast of
the cyclone's eye, he said.

Cyclone-spawned tornadoes are not fundamentally different from
the tornadoes that form in the Great Plains. One difference is
that the former tend to be less powerful, usually not exceeding a
rating of EF2 on the
Enhanced Fujita scale. Secondly, twisters that form in the
Plains, like the
tornado that struck Moore, Okla., get all of their
ingredients from separate places. In the case of the Oklahoma
tornado outbreak, for example, the warm air came north from the
Gulf of Mexico, while the cold air came south from Canada. In the
case of hurricanes, however, they provide all the required
components for twisters themselves.